Under appropriate conditions, differentiated plant tissues dedifferentiate and form calli (groups of dedifferentiated cells) after undergoing cell divisions. Depending on the conditions, calli can further redifferentiate to regenerate complete plant bodies. The ability of such differentiated cells or dedifferentiated cells to regenerate individual bodies is called totipotency, and this was initially demonstrated in the 1930s to 1950s in cultivation studies of tobacco, tomatoes, and such. Tissue culture techniques are based on this totipotency, and have been widely utilized, particularly in the field of plant breeding. For example, tissue culture techniques have been used in the production of new varieties by cell fusion and ovule culture, shortening the number of years taken for breeding and fixing of hereditary character. In recent years, tissue culture techniques have become essential for molecular breeding and basic research on plants as key techniques in artificial gene transfer (transformation methods) aimed at the functional analysis of genes.
Totipotency is generally thought to be an ability possessed by all plants. In fact, depending on the plant type, variety or organ, it is known to be easy for some plants to exhibit this ability, and difficult for others. Compared to dicotyledonous plants, the tissue culture and regeneration of monocotyledonous plants including major crops such as rice, wheat, and corn is difficult, and therefore repeated trial and error is necessary for analyses involving cultivation, including transformation methods. In rice a relatively simple culturing system has been established using the ripe seeds of specific varieties, however varieties with sufficient regenerative ability are limited. In particular, palatable varieties such as Koshihikari and Sasanishiki, and the IR line varieties widely cultivated in the tropics have low regenerative abilities, and regeneration of a plant body by tissue culturing is difficult. Improving the regenerative ability of these varieties would not only be useful for selective breeding and study of gene characteristics, but might also lead to elucidation of the mechanism of the regenerative process. In addition, the regenerative ability of other unculturable plant species and varieties might also be improved.
Furthermore, in recent years a large number of genetically modified agricultural products (GMOs) have been developed, and their planted area is increasing year by year. At the same time, many consumers are worried about their safety. The major concern in discussions on the safety of GMOs is their incorporation of antibiotic-resistance genes. Therefore, development of transformation methods that do not use antibiotic-resistance genes will ease existing consumer concern over GMOs, and at the same time may also be advantageous to researchers as simple transformation methods that do not require expensive antibiotics.